1. Field of the Invention
The invention relates to the fabrication of integrated circuits and to a process for depositing dielectric layers on a substrate and the structures formed by the dielectric layers.
2. Description of the Related Art
One of the primary steps in the fabrication of modern semiconductor devices is the formation of metal and dielectric layers on a substrate by chemical reaction of gases. Such deposition processes are referred to as chemical vapor deposition or CVD. Conventional CVD processes supply reactive gases to the substrate surface where heat-induced or energy-enhanced chemical reactions take place to produce a desired layer.
Semiconductor device geometries have dramatically decreased in size since the devices were first introduced several decades ago. Since then, the number of devices that will fit on a chip doubles every two years. Tomorrow's plants soon will be producing devices with smaller geometries.
To further reduce the size of devices on integrated circuits, the industry is using conductive materials having low resistivity and insulators having low dielectric constants (dielectric constants of less than 4.0) to reduce the capacitive coupling between adjacent metal lines. One such low dielectric constant (low k) material comprises silicon, oxygen, and carbon and may be deposited as a dielectric material in fabricating damascene features. One conductive material having a low resistivity is copper and its alloys, which have become the material of choice for sub-quarter-micron interconnect technology because copper has a lower resistivity than aluminum, (1.7 μΩ-cm compared to 3.1 μΩ-cm for aluminum), a higher current and higher carrying capacity. These characteristics are important for supporting the higher current densities experienced at high levels of integration and increased device speed. Further, copper has desirable thermal conductivity and is available in a highly pure state.
Unfortunately, copper is difficult to etch and achieve a precise pattern. Etching with copper using traditional deposition and etch processes for forming interconnects has been less than satisfactory. Thus, new methods for manufacturing interconnects containing copper and low k dielectric materials are being developed.
One method for forming vertical and horizontal interconnects is by using damascene or dual damascene method. In the damascene method, one or more dielectric materials, including low k dielectric materials, are deposited and pattern etched to form the vertical interconnects (vias) and horizontal interconnects (lines). Conductive materials such as copper and barrier layer materials used to prevent diffusion of copper into the surrounding low k dielectric are then inlaid into the etched pattern. Any excess copper and barrier layer materials external to the etched pattern remaining on the field of the substrate are then removed.
However, low k dielectric materials are often porous and susceptible to interlayer diffusion of conductive materials which can result in the formation of short-circuits and device failure. A dielectric barrier layer material is used in copper damascene structures to reduce or to prevent interlayer diffusion. Traditional dielectric barrier layer materials often have high dielectric constants of 7 or greater. The combination of a high k dielectric material with surrounding low k dielectric materials results in dielectric stacks having a higher than desired dielectric constant.
Additionally, forming damascene structures requires the use of lithographic processes. For example, in process sequences using conventional lithographic techniques, a layer of energy sensitive resist is formed over a stack of material layers on a substrate. Many of the underlying material layers are reflective to ultraviolet light. These reflections can distort the dimensions of features such as lines and vias that are formed in the energy sensitive resist material. This is troublesome when using 193 nm radiation with reflective metal layers. One technique proposed to minimize reflections from an underlying material layer uses an anti-reflective coating (ARC). The ARC is formed over the reflective material layer prior to resist patterning. The ARC suppresses the reflections of the underlying material layer during resist imaging, providing accurate pattern replication in the layer of energy sensitive resist.
However, conventional ARC materials contain nitrogen, including silicon nitride and titanium nitride. Nitrogen in the ARC layer may chemically alter the composition of the photoresist material. The chemical reaction between nitrogen and the photoresist material is referred to as photoresist poisoning. The altered photoresist material may not be lithographically patterned as expected and result in imprecisely formed features in the photoresist material or excessive photoresist residue remaining on the substrate surface after photoresist patterning, both of which can detrimentally affect subsequent processes, such as etching processes. For example, nitrogen may neutralize acid near a photoresist and ARC interface and result in residue formation, known as footing, which can further result in curved or rounded aspects at the interface of the bottoms and sidewalls of features rather than desired right angles.
Additionally, low k materials are susceptible to surface defects or feature deformation during polishing and removal of conductive materials under conventional polishing processes. One solution to limit or reduce surface defects and deformation is to deposit a hardmask over the exposed low k materials prior to patterning and etching feature definitions in the low k materials. The hardmask is resistant to damage and deformation. The hardmask also protects the underlying low k materials during subsequent material deposition and planarization or material removal processes to reduce defect formation and feature deformation.
Also, conventional hardmask materials do not have sufficient selectivity to oxide or metal during polishing, which may result in premature removal of the hardmask and expose the underlying material to the process. The exposed underlying low k dielectric material may be damaged and result in surface defects and feature deformation. Additionally, hardmasks and ARC materials may remain as part of the structure after the underlying dielectric layer is etched and contribute to the structure's overall dielectric constant. Conventional hardmask materials often have high dielectric constants of 7 or greater, which can produce dielectric stacks having a higher than desired dielectric constant. Current hardmask materials have not satisfactorily produced both low k material and sufficient polishing selectivity to be used in damascene fabrication.
Hardmask and ARC material formed from conventional material may exhibit a porous surface. The small holes are referred to as pinholes. The pinholes can be formed completely through the ARC layer thereby exposing photoresist material deposited on the ARC layer to material underlying the ARC layer, such as silicon nitride. Nitrogen from silicon nitride or other nitrogen containing material may diffuse through the ARC layer and chemically alter the composition of the photoresist material and result in photoresist poisoning.
Further, as the device size shrinks to 0.13 um or below and chipmakers migrate to dual damascene process for faster and higher-level performance, new lithographic challenges arise. As the photolithographic patterning wavelength is reduced to 193 nm for forming features of 0.13 um or below, new photoresists (PR) are being developed to work in conjunction with photolithography at a wavelength of 193 nm. Amine radicals (NH2) from current dielectric antireflective coatings, such as SiOxNy, neutralize the acid catalyst in 193 nm PR. Footing results because the neutralized portion of PR is insoluble in developer. The use of the shorter 193 nm wavelength also results in the substrate being more reflective, and increases the difficulty in controlling critical dimensions variations (CD swing), and effective photolithographic processing requires substrate reflectivity below 1 percentage for the shorter wavelengths.
Dual damascene process presents additional challenges for control of CD swing. For example, Al interconnect processing consists of blanket Al layer deposition, photolithography process to pattern an Al layer, and dielectric gap fill. The high reflectivity and absorption of Al layers of the DUV wavelength does not allow any DUV light to transmit through the Al layers. Thus, the various structures underlying an Al layer do not contribute to any reflection above the Al layer. In contrast, for dual damascene process, the oxide-like low k dielectric is transparent at the patterning wavelength so that the substrate reflection varies depending on the underlying structure. Additionally, anti-reflective layers require dual damascene applications to have sufficient interlayer adhesion with low k inter-metal dielectric materials (IMD). Current anti-reflective coating materials and processes have not been satisfactory in meeting these requirements.
Therefore, there remains a need for an improved process and material for forming dielectric materials suitable as anti-reflective coatings or hardmasks with a satisfactory etching selectivity for damascene applications.